1. Field of the Invention
The present invention generally relates to a sheet processing apparatus to bind together a bundle of sheets and an image forming system including the sheet processing apparatus and an image forming apparatus, such as a copier, a facsimile machine, a printer, or multifunction machine capable of at least two of these functions.
2. Description of the Background Art
There are sheet processing apparatuses, so-called finishers or post-processing apparatuses, that align a bundle of sheets (hereinafter “a sheet bundle”) output from an image forming apparatus and bind the sheet bundle with metal staples. Such sheet processing apparatuses can automatically staple a number of sheet bundles on which images are formed and are widely used for convenience and efficiency thereof.
Additionally, there are hand-held staplers, so-called staple guns or powered staplers, capable of binding sheets without metal staples. For example, there are hand-held staplers that press multiple sheets with a tooth form so that fibers of the sheets tangle with each other and thereby tie the sheets together, or bind the sheets together using other types of processing such as half blanking, lancing, bending, and inserting. Such binding tools can reduce consumption of consumables, make recycling easier, and be effective to save resources because sheets bound by them are free of metal staples and can be directly put through a shredder.
It is to be noted that, hereinafter clamp binding refer to a binding method that involves pressing multiple sheets with a tooth form to tie the sheets, thereby causing fibers of the sheets to tangle with each other. The portions where the fibers are tangled are referred to as “clamping marks”. For example, JP-S36-13206-Y discloses a hand-held stapler capable of clamp binding, and JP-S37-7208-Y discloses a hand-held stapler that makes cut holes in sheets, bends cut portions, and inserts the cut portions into the cut holes.
Use of clamp binding in sheet processing apparatuses is expected to increase owing to the above-described advantages. The strength of binding by clamp binding, however, is lower than that attained by metal staples, and, if the sheet bundle is handled roughly, the clamping marks might be loosened, allowing the sheet to come off from the sheet bundle. To enhance the binding strength of clamp binding, for example, JP-2004-15537-A proposes changing the number or arrangement of tooth of the tooth form depending on variables relating to the sheets bound thereby.